Sex smell lures 'vampire' to doom

A synthetic "chemical sex smell" could help rid North America's Great Lakes of a devastating pest, scientists say.

US researchers deployed a laboratory version of a male sea lamprey pheromone to trick ovulating females into swimming upstream into traps.

The sea lamprey, sometimes dubbed the "vampire fish", has parasitised native species of the Great Lakes since its accidental introduction in the 1800s.

The work is reported in Proceedings of the National Academy of Sciences.

The Great Lakes on the US-Canada border support recreational fishing worth billions of dollars a year, which the lampreys would wreck but for a control programme costing about $20m annually.

This is thought to be the first time that pheromones have been shown to be the basis of a possible way of controlling animal pests other than insects.

"There's been extensive study of pheromones in animals and even in humans," said lead researcher Weiming Li from Michigan State University in East Lansing, US.

It was one of the worst things to hit the Great Lakes in the history of European settlement

Marc Gaden

"But most researchers have presumed that as animals get more complex, their behaviour is regulated in a more complex way, not by just one pheromone," he told BBC News.

Professor Li's team released the synthetic version of a lamprey hormone from a trap placed in a stream where lampreys come to breed.

Females scenting it would swim vigorously upstream until they found the source, some becoming trapped in the process.

Death wish

The sea lamprey's natural life cycle takes it from birth in a stream to adulthood in the ocean, where it gains its vampirical appellation.

Circular jaws lock on to another, larger fish, and a sharp tongue carves through its scales.

From then on the lamprey feeds on the blood and body fluids of its temporary host, often killing it in the process.

Eventually, the satiated lampreys - both males and females - find a suitable stream to swim up, breed and die.

The female lampreys were lured into traps on the stream

Unlike salmon, which seek out the stream they were born in, lampreys appear willing to take any stream indicating a suitable breeding place; and perhaps pheromones play a role in identifying streams worth selecting.

In their native Atlantic Ocean, their numbers are controlled by predation; but in the Great Lakes they have no predators.

They first appeared in the 1800s after completion of the Erie Canal linking the lakes to New York.

Colonisation was completed a century later when other canals provided unfettered access to the upper lakes.

What followed was decimation of native fish.

"It was one of the worst things to hit the Great Lakes in the history of European settlement," said Marc Gaden from the Great Lakes Fishery Commission (GLFC), the body responsible for controlling the lamprey problem.

"Before it, we had a thriving fishery largely dependent on native fish such as the lake trout... but by 1940 they had colonised thousands of streams and fishermen were beginning to see the devastation."

Getting fresh

Many fish can survive only in fresh water or only in the oceans - or, like salmon, have a set migration between the two - but the lamprey appears to have thrived on its move from the saline Atlantic to the fresh environs of the five lakes.

Each individual devours a total weight of up to 20kg of trout or other host fish during its parasitic lifetime.

The GLFC has established a complex set of control measures, including dusting the streams with pesticides specific to the lamprey, building barriers to block their upstream migration, and releasing sterile males to reduce breeding.

"Why we're so enthusiastic about the pheromone work is that we see it as another tool in the arsenal," said Dr Gaden.

"We see it as away of tricking these spawning lampreys, and then you can do things to manipulate their behaviour in ways that would work against them - for example you could lure them into streams without suitable spawning habitat, or just into traps."

Professor Li's team is now planning a larger experiment, using the pheromone to trap female lampreys in 20 streams feeding into the lakes, which will take three years to complete.

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